1. Field of the Invention
The present invention relates to an analog-to-digital (AD) conversion controller that controls an AD converter that converts received signals into digital signals by performing an AD conversion at a predetermined sampling timing in an optical receiving device that receives optical signals, an optical receiving device that include the AD conversion controller, an optical receiving method, and a waveform-distortion compensating device that equalizes a waveform of received signals that are subjected to the AD conversion in the AD converter.
2. Description of the Related Art
In recent years, a coherent optical communication has become focused as means for satisfying a demand for a high-speed network with a large capacity. This is because a coherent optical communication has an excellent resistance against optical noises and is more immune to the influence of amplification relay, and thus poses less restriction on the transmission distance.
The transmission distance of the optical communication is restricted due to noises and waveform distortion. The noise can be reduced by the excellent optical noise resistance of the coherent optical communication. On the other hand, the waveform distortion is a problem, for example, dispersion properties of a transmission channel, in particular wavelength dispersion poses a problem. The wavelength dispersion is a phenomenon in which group delay of optical signals changes according to frequency.
In the coherent optical communication, the phase data as well as the strength of the optical signal is not lost in detection and conversion into electric signals. Therefore, the effect of wavelength dispersion on the signals in an optical region can be easily compensated by a linear circuit at the level of electric signals obtained as a result of detection and conversion. In other words, when compared with a commonly used system that directly detects the waves by extracting only an optical strength by square-law detection, the coherent optical communication has a high capability of electric waveform distortion compensation to compensate the waveform distortion at the level of the electric signals.
Thus, the high electric waveform distortion compensation capability can be obtained in the coherent optical communication. For realizing the electric waveform distortion compensation, detecting and compensating a relative optical phase difference between a local light and the optical signals is necessary. One method to realize this is disclosed in Kazuhiro Katoh, Kazuro Kikuchi, “Unrepeated 210-km Transmission with Coherent Detection and Digital Signal Processing of 20-Gb/s QPSK Signal”, Optical Fiber Communication Conference & Exposition 2005, Year 2005. According to which, the optical signals are coherently received and AD conversion is carried out. The converted digital signals are further accumulated in a storage unit and, the optical phase difference between the optical signal and a local light is calculated based on the accumulated digital signals through digital signal processing, and the optical phase difference is compensated to detect the optical signal.
In Timo Pfau et al., “1.6 Gbit/s Real-Time Synchronous QPSK Transmission with Standard DFB Lasers”, European Conference On Optical Communication 2006, Year 2006, a method is disclosed in which the optical signals of 1.6 gigabits per second (Gbit/s) are coherently received and AD conversion is carried out. Based on the optical phase difference between the optical signal and a local light calculated by a processor based on the AD converted digital signals, the optical phase difference is canceled and transmission is carried out in real time.
Thus, the coherent optical communication has high electric waveform distortion compensation capability, and as a method for compensating the electric waveform distortion, various techniques are disclosed. For example, Japanese Patent Application Laid-open No. H8-163027 (paragraph [0005], FIG. 1) discloses an optical signal receiving processing circuit which includes a delay unit causing the received optical signals or the electric signals obtained through conversion of the received optical signals by a photoelectric converter to delay, a coefficient multiplier that multiplies each delayed output signal by a coefficient, an adder that adds the output signals of each coefficient multiplier, and a coefficient operator that calculates the coefficient mentioned earlier.
Further, Japanese Patent Application Laid-open No. 2003-258606 (paragraphs [0020] to [0031], FIG. 1) discloses an optical signal receiving processing circuit in which a delay time is set in a level-shift circuit and an amplifier is formed of an exclusive OR gate. An output node is shared by amplifiers and a common load resistance ZL is arranged at the common node. All output electric currents are added at the common load resistance ZL and the added electric currents are converted into a voltage. By using this optical signal receiving processing circuit, compensation can be carried out simultaneously without discrimination between the wavelength dispersion and polarized wave dispersion.
Still further, Japanese Patent Application Laid-open No. 2000-292263 (paragraphs [0007] and [0008], FIG. 1) discloses an optical receiver that includes a photoelectric converter, an equalizer, and a microprocessor. The equalizer is connected to at least one distortion detector, and both the distortion detector and the equalizer are connected to a common controlling unit via the microprocessor. In this optical receiver, parameters of polarization mode dispersion are directly measured. A measurement result can be used for analysis of the equalizer. Furthermore, the parameters of the polarization mode dispersion can be measured with respect to the input signals irrespective of the modulation.
Still further, Japanese Patent Application Laid-open No. 2002-171203 (paragraph [0012], FIG. 1) discloses an echo canceller that includes an initial value data storage unit that stores therein multiple initial value candidates used as an initial value in an echo canceling process, an initial value determining unit that specifies an optimum initial value upon obtaining residual signals when the respective initial value candidate is applied from the initial value candidates stored in the initial value data storage unit, an internal status updating unit that updates an internal status amount by using the initial value specified in the initial value determining unit as the initial value, and an adaptive filter that updates, based on the internal status amount updated by the internal status updating unit, a filter coefficient and creates an echo replica.
In general, for avoiding a loss of the strength and phase data of the optical signals in AD conversion, a sampling rate of the AD conversion must be sufficiently higher than a symbol rate of the optical signals. Furthermore, executing an appropriate arithmetic process is necessary. The above-mentioned technique disclosed in Kazuhiro Katoh, Kazuro Kiuchi, “Unrepeated 210-km Transmission with Coherent Detection and Digital Signal Processing of 20-Gb/s QPSK Signal”, Optical Fiber Communication Conference & Exposition 2005, Year 2005 does not realize real-time transmission because the sampling rate of the AD conversion is not sufficiently higher than the symbol rate of the optical signals, and an exceptionally complicated arithmetic operation must be performed for obtaining data of the optical signals. On the other hand, the technique disclosed in Timo Pfau and others, “1.6 Gbit/s Real-Time Synchronous QPSK Transmission with Standard DFB Lasers”, European Conference On Optical Communication 2006, Year 2006, though realizing a real-time transmission, achieves a bit rate only as high as 1.6 Gbit/s.
For example, for realizing the transmission in real time with high-speed optical signals exceeding 20 giga-symbols per second (Gsymbol/s), either an extremely high-speed sampling rate or a complex calculation or both are necessary. Because of technical constraints, a cost, and a space, realizing the transmission in real time is very difficult.
Even if an AD converter that realizes a high-speed sampling rate can be implemented, a processing load on a processor processing the digital signal in a subsequent step increases. Then, a circuit scale of the processor or a drive frequency must be increased. Thus, the technology is scarcely useful because of the technical constraints, the cost, and the space.
In other words, because of various constraints, the sampling rate of the AD converter needs to be low as far as possible. Therefore, the sampling rate of the AD conversion must be set to a value close to the symbol rate of the optical signals. In other words, the sampling rate of the AD conversion must be set equivalent to or at most several times the symbol rate of the optical signals.
However, when the sampling rate of the AD conversion is lowered as far as possible and becomes lower than several times the symbol rate of the optical signals, a sampling timing of the AD conversion must be synchronized with a symbol of the received optical signals substantially. If a sampling frequency of the AD conversion shifts from the timing of the symbol, data included in the received optical signals cannot be retrieved by a high signal-to-noise ratio, and an error rate increases.
In the conventional technique as represented by Japanese Patent Application Laid-open No. H8-163027 (paragraph [0005], FIG. 1), the coefficient to be multiplied with the input signals for compensating the waveform distortion is calculated according to the level of output signals of the optical signal receiving processing circuit. Therefore, the technique allows for automatic control to adjust to the changes in a transmission channel caused over time or changes caused by temperature variation. However, compensation of waveform distortion cannot be carried out swiftly in an early stage of the optical signal receiving process.
In the conventional technology as represented by Japanese Patent Application Laid-open No. 2003-258606 (paragraphs [0020] to [0031], FIG. 1), even if resolution power of the delay time can be easily set at will and a large output amplification of the output signals can be easily secured, it is necessary to implement a complex circuit in a connection circuit. However, mounting the complex circuit is not easy.
In the conventional technique as represented by Japanese Patent Application Laid-open No. 2000-292263 (paragraph [0007] and [0008], FIG. 1), even if the waveform distortion can be compensated by detecting the waveform distortion in high-speed and analyzing a detection result by the equalizer, it is necessary to implement the complex circuit in the connection circuit similarly as in the conventional technology represented by Japanese Patent Application Laid-open No. 2003-258606 (paragraphs [0020] to [0031], FIG. 1), and mounting of the complex circuit is not easy.
The conventional technique as represented by Japanese Patent Application Laid-open No. 2002-171203 (paragraph [0012], FIG. 1) is related to the echo canceller. Even if the technique is applied for compensating the waveform distortion, executing a complex algorithm for compensating the waveform distortion is necessary, and therefore swift waveform distortion compensation is difficult to perform.
Further, even if the conventional techniques mentioned above are combined, with respect to the digital signals after AD conversion, the waveform distortion cannot be compensated in the high-speed in a simple structure. For example, when a redundantly structured transmission channel is switched from an operating system to a standby system, compensating the waveform distortion of a new transmission channel takes time. Therefore, the transmission channel cannot be rapidly switched, and communication remains to be cut over a predetermined period of time.